EOM inside an optical cavity

A passive optical frequency comb uses an electro-optic phase modulator (EOM) to superimpose a microwave frequency onto an optical carrier frequency, producing a comb of equally-spaced modulation sidebands. Typically, significant energy is transferred only into the first-order sidebands, and the power in the higher-order sidebands falls off rapidly with increasing order number.

To enhance the power in the sidebands, and hence the number of modes which can be detected, the EOM is placed inside an optical cavity which is resonant with both the carrier frequency and all the sidebands.

The NPL 1.5 micron passive combThis means that the free spectral range of the optical cavity must be equal to, or a subharmonic of, the modulation frequency. The sidebands generated from the optical carrier then pass back through the EOM and generate secondary sidebands, which are also resonant with the cavity and in turn generate further sidebands. In this way, a wide comb of modes spaced by exactly the modulation frequency can be produced. The accuracy of the frequency intervals between these modes is determined only by the accuracy of the microwave oscillator used to drive the EOM, which can be referenced to a frequency standard such as a hydrogen maser.

The limit of the frequency difference at which beats can be detected between comb modes and a second laser depends on the power of the sidebands. The power in a particular high order sideband increases as either the finesse of the optical cavity or the modulation index is increased. The span of this type of optical frequency comb is ultimately limited by the dispersion of the cavity elements, in particular, dispersion in the electro-optic crystal. However, passive combs have successfully been used to bridge frequency gaps of several THz in the optical region of the spectrum.

At NPL we have developed a passive optical frequency comb at 1.5 microns for applications in the optical telecommunications region of the spectrum. This comb uses an electro-optic modulator operating at a frequency of 10 GHz to generate a comb spanning up to 2.5 THz. To provide frequency traceability, the laser driving the optical frequency comb is offset-locked to an acetylene-stabilised diode laser.